PGP word list

The PGP Word List (also called a biometric word list for reasons explained below) is a list of words for conveying data bytes in a clear unambiguous way via a voice channel. They are analogous in purpose to the NATO phonetic alphabet used by pilots, except a longer list of words is used, each word corresponding to one of the 256 unique numeric byte values.

History and structure

The PGP Word List list was designed in 1995 by Patrick Juola, a computational linguist, and Philip Zimmermann, creator of PGP. The words were carefully chosen for their phonetic distinctiveness, using genetic algorithms to select lists of words that had optimum separations in phoneme space. The candidate word lists were randomly drawn from Grady Ward's Moby Pronunciator list as raw material for the search, successively refined by the genetic algorithms. The automated search converged to an optimized solution in about 40 hours on a DEC Alpha, a particularly fast machine in that era.

The Zimmermann/Juola list was originally designed to be used in PGPfone, a secure VoIP application, to allow the two parties to verbally compare a short authentication string to detect a man-in-the-middle attack (MiTM). It was called a biometric word list because the authentication depended on the two human users recognizing each other's distinct voices as they read and compared the words over the voice channel, binding the identity of the speaker with the words, which helped protect against the MiTM attack. The list can be used in many other situations where a biometric binding of identity is not needed, so calling it a biometric word list may be imprecise. Later, it was used in PGP to compare and verify PGP public key fingerprints over a voice channel. This is known in PGP applications as the "biometric" representation. When it was applied to PGP, the list of words was further refined, with contributions by Jon Callas. More recently, it has been used in Zfone and the ZRTP protocol, the successor to PGPfone.

The list is actually comprised of two lists, each containing 256 phonetically distinct words, in which each word represents a different byte value between 0 and 255. Two lists are used because reading aloud long random sequences of human words usually risks three kinds of errors: 1) transposition of two consecutive words, 2) duplicate words, or 3) omitted words. To detect all three kinds of errors, the two lists are used alternately for the even-offset bytes and the odd-offset bytes in the byte sequence. Each byte value is actually represented by two different words, depending on whether that byte appears at an even or an odd offset from the beginning of the byte sequence. The two lists are readily distinguished by the number of syllables; the even list has words of two syllables, the odd list has three. The two lists have a maximum word length of 9 and 11 letters, respectively. Using a two-list scheme was suggested by Zhahai Stewart.

Word Lists

Examples

Each byte in a bytestring is encoded as a single word. For example, the least significant byte (i.e. byte 0) is considered "even" and is encoded using the PGP Even Word table. The next most significant byte (i.e. byte 1) is considered "odd" and is encoded using the PGP Odd Word table. This process repeats until all bytes are encoded. Thus, "E582" produces "topmost Istanbul", whereas "82E5" produces "miser travesty".

A PGP public key fingerprint that displayed in hexadecimal as
:E582 94F2 E9A2 2748 6E8B
:061B 31CC 528F D7FA 8919
would display in PGP Words (the "biometric" fingerprint) as
:topmost Istanbul Pluto vagabond
:treadmill Pacific brackish dictator
:goldfish Medusa afflict bravado
:chatter revolver Dupont midsummer
:stopwatch whimsical nightbird bottomless

The order of bytes in a bytestring is a topic discussed at length in computer science and engineering, and is beyond the scope of this article. This is often referred to as Endianness.

Other word lists for data

There are several other word lists for conveying data in a clear unambiguous way via a voice channel:
* the NATO phonetic alphabet maps individual letters and digits to individual words
* the S/KEY system maps 64 bit numbers to 6 short words of 1 to 4 characters each from a publicly accessible 2048-word dictionary. The same dictionary is used in RFC 2289.
* the Diceware system maps 5 base-6 random digits (almost 13 bits of entropy) to a word from a dictionary of 7,776 unique words.
* FIPS 181: Automated Password Generator converts random numbers into somewhat pronounceable "words".
* mnemonic encoding converts 32 bits of data into 3 words from a vocabulary of 1626 words. [ [http://www.tothink.com/mnemonic/ mnemonic encoding] ]

References

Patrick Juola & Philip Zimmermann. [http://www.mathcs.duq.edu/~juola/papers.d/icslp96.pdf "Whole-Word Phonetic Distances and the PGPfone Alphabet" (1996)] . Proceedings of the International Conference of Spoken Language Processing (ICSLP-96)

Patrick Juola. [http://www.mathcs.duq.edu/~juola/papers.d/pgpfonenemlap.ps "Isolated Word Confusion Metrics and the PGPfone Alphabet" (1996)] . Oxford University, Dept. of Experimental Psychology. Proceedings of New Methods in Language Processing 2, Ankara, Turkey, September, 1996.

Copyright

This material is copyrighted under a copyright owned by PGP Corporation. They have now granted a license under the GNU Free Documentation License. (per Jon Callas, CTO, CSO PGP Corporation, 4-Jan-2007)